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I am working on a project that requires the use of USB 3.0. There are two enclosures used in the project, therefore 3 cables are required.

  • Cable 1: PC to enclosure 1 panel mount connector (1m)
  • Cable 2: Cable between enclosures (1.5m)
  • Cable 3: Enclosure 2 panel mount connector to device (1.25m)

This means the total cable length is 3.75m, which is above the 3m recommendation. My issue is finding a USB repeater to go between these cables.

I could possibly combine cables 2&3 together to make one longer cable, however I need a right angle USB Type A connector to interface with the device as a straight connector clashes.

Would anybody know if I could get away without using a repeater or have any recommendations?

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  • \$\begingroup\$ Are you using USB 3.0 superspeed, or can you get away with reducing the speed? Really, there's not much anyone can guarantee without knowing the exact cables you're using, but it might work. Then again, it might not. \$\endgroup\$ – Hearth Jul 1 '19 at 14:12
  • \$\begingroup\$ @Hearth, I could reduce the speed. The device is a scientific camera which can be sensitive to interference so as long as all cables are shielded there shouldn't be an issue. i was thinking of using this cable for cable 1 link I was planning on using a repeater on cable 2, however, I can't find any under 3m \$\endgroup\$ – cath001 Jul 1 '19 at 14:15
  • \$\begingroup\$ Also, can you not use shorter cables for cable 1 and cable 3? It would seem like a meter plus would be excessive for things internal to the enclosure, unless it's very large equipment. \$\endgroup\$ – Hearth Jul 1 '19 at 14:53
  • \$\begingroup\$ Do I get it correctly that your enclosures don't touch any USB data? What about the traces within the enclosures? How big they are? \$\endgroup\$ – Ale..chenski Jul 1 '19 at 23:40
  • \$\begingroup\$ There are moving modules inside the enclosures therefore the USB cables have to go through cable chains and hence require longer length to allow the modules to move without pulling on the cable \$\endgroup\$ – cath001 Jul 2 '19 at 15:09
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This means the total cable length is 3.75m, which is above the 3m recommendation.

This is a misconception. The "3m recommendation" is based on use of generally-practiced consumer-grade raw cables having two USB-defined connectors. In theory the channel length is limited only by ability of the cable to propagate signals with differential loss of less than -25dB at 7.5 GHz. If you can make a 20-m cable from very expensive materials and/or finger-size thick that meets this specification, the USB 3.x link should work just fine.

However, you are planning to break the link into 4 segments, and thus introduce 4 extra connectors. Connectors are usually the worst to maintain characteristic impedance, and all board-to-connector (or cable-to-connector attachment) fan-out has horrible variability and usually introduces impedance mismatches, and heavy inter-symbol interference results. This is the primary reason why USB standard strongly discourages the use of passive cable extenders, the necessary quality of interconnect is simply not achievable in consumer-grade production. So your "passive" approach will require the use of very expensive military-grade connectors and careful design of internal PCBs or whatever do you plan to use inside your enclosures.

As you mention, the other option is to use either a re-driver (I would use two of them, at the output of each enclosure), or a low-port-count USB 3.0 hub. The use of two classic analog re-drivers in series is however questionable, since each redriver will introduce extra jitter, which might kill the link. The solution is in new class of USB devices called re-timers, but they are fairly new devices, I am not sure if they already hit the market, and have no experience with them.

In all above cases the success is highly questionable or will take a lot of efforts and expensive equipment to tune the link. I would still try to find a way to connect your PC with the cam with one-segment active extender, preferably based on fiber-optics, just like the cables offered by Corning Communication.

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